The present invention pertains to the purification of contaminated groundwater and, more specifically, to the purification of groundwater having hexavalent chromium.
Recently, zero-valent iron filings have been identified as a suitable constituent for use in permeable reaction wall technology to remove certain contaminants from groundwater. This technology uses a form of passive groundwater remediation which typically involves placing a reactive wall or barrier in the flow path of contaminated groundwater. According to this technology, iron filings are typically mixed with sand to allow for a suitable permeability of the reactive barrier. The wall thickness and permeability are selected to provide for an adequate residence time of the contaminated groundwater within the barrier to reduce certain contaminants in the water to a desired level, such as below drinking water levels. The wall is typically oriented perpendicular to the flow path of the groundwater.
In some cases, a xe2x80x9cfunnel and gatexe2x80x9d configuration may be used. The xe2x80x9cfunnelxe2x80x9d consists of a sealable joint sheet pile or slurry which directs contaminated water to the iron wall or a xe2x80x9cgatexe2x80x9d and also prevents untreated groundwater from flowing around the gate. The impermeable funnels allow containment and treatment of a contaminant flow path (or plume) without constructing an iron wall across the plume""s entire width.
An advantage of this type of groundwater remediation is that there are very few operating and maintenance costs after the reactive barrier is installed. Groundwater monitoring before and after installation of the reactive barrier is required in order to verify the effectiveness. Permeable barrier remediation technology is a growing field and is anticipated to be a major cost-effective groundwater remediation methodology of the future.
It has been reported that the present cost of zero-valent iron in a particle size suitable for a permeable reactive wall is approximately $400 per ton. If the volume of the wall is large, the cost of the reactive zero-valent iron can be considerable. Moreover, iron filings may have other elements, such as sulfur, selenium, arsenic, cadmium, lead, copper, and mercury, in a form which allows them to leach relatively easily to the environment. In addition, iron filings tend to absorb carbon dioxide from air or tend to be reactive with carbonate ions or carbonic acid from aqueous solution to form insoluble carbonates, thereby reducing the permeability by the plugging action of carbonates. Therefore, it is desirable to identify a constituent which might be used as a permeable barrier to remediate or purify contaminated groundwater. Preferably, such a constituent is not as costly as iron filings, does not contain certain elements in a readily leachable form, and does not absorb carbon dioxide from air or react with carbonate or carbonic acid.
In view of its purposes, the present invention provides a method for purifying contaminated water comprising passing contaminated water through a porous bed comprising steel slag. It has been found that the porous bed may consist solely of steel slag, with no other constituent or filler, such as sand, mixed with the steel slag. Preferably, the method involves first providing an underground barrier comprising a porous bed of steel slag in a flow path or plume of contaminated groundwater then allowing the groundwater to pass through the barrier to purify the groundwater. The slag can be used as formed, with no further grinding needed, and preferably is sieved such that the slag used has particles with a median diameter within the range of about one-eighth of an inch to one inch and preferably from about one-quarter of an inch to three-quarters of an inch. The present invention is particularly well-suited to removing hexavalent chromium from contaminated groundwater.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.
The present invention is directed to a method which utilizes steel slag as a permeable barrier within a flow path or plume of contaminated groundwater to purify the groundwater. Steel slag is the cooled, solidified co-product obtained in the process for making steel and is formed when steel-containing ores and/or scrap are treated and/or purified in a steel making furnace. In general, steel slag is itself a non-metallic product, consisting primarily of silicates and aluminosilicates of calcium and other bases that develop in a molten condition during steel making. The particular constituents and concentrations of those constituents vary depending on the process conditions of the steel furnace from which the slag is removed. Nonetheless, it is believed that any steel slag can be used in the present invention. Typically, steel slag has generally the following composition, although it varies as stated above:
Steel slag can also include other constituents not listed above such as Fe2O3, K2O, Na2O, and r2O3.
The median particle size of the steel slag should be within a particular range. For example, if the median particle size is too large, then the overall surface area available for reaction with the contaminants is decreased, thereby decreasing the efficiency of the permeable wall. On the other hand, if the median particle size is too small, such as less than about one-eighth of an inch, then the fine particles might tend to cementitiously harden when formed into a wall and thereby become relatively impermeable to groundwater. The formation of an impermeable wall would be severely detrimental to the purification system in that the contaminated groundwater would find another route around the barrier and therefore would remain contaminated.
The steel slag used for the permeable wall may have a fairly broad distribution in particle size and can include the presence of very fine particles and large particles. In the case of a broad distribution, the presence of large particles would tend to limit the formation of regions of cementitious hardening otherwise caused by the small particles. The use of steel slag having a broad distribution is desirable for the reason that such slag requires less processing (i.e., little or no separation) before use as a permeable wall. On the other hand, the steel slag used for the permeable wall may have a fairly narrow distribution in particle size. The use of steel slag having a narrow distribution is desirable for the reason that such slag tends to have a more homogenous reactivity throughout the permeable wall.
Generally, the steel slag used should have particles with a median diameter of about one-eighth of an inch to one inch, and preferably between about one-quarter of an inch and three-eighths of an inch. To obtain steel slag particles of this size, the formed steel slag need not be crushed or ground, but can merely be sieved in a known way to achieve a set of particles having an average diameter within these ranges. In some cases, steel slag as formed can be used.
It has been found that the steel slag generally has a sufficiently high permeability (e.g., about 4.5xc3x9710xe2x88x922 cm/sec) to be the sole constituent in the porous bed. Moreover, it has been found that steel slag retains this high permeability even after water has passed through it. As mentioned above, in iron filings technology, the iron filings are typically mixed with sand before placed in a plume of contaminated groundwater. Because the permeable wall of the present invention is entirely the reactive material, the efficiency of the wall, measured as reactive sites per unit weight of wall, is increased as compared to a wall of iron filings.
The method of the present invention is used for purifying contaminated groundwater. As demonstrated below, steel slag has been shown to remove hexavalent chromium and trichloroethylene (TCE), two contaminants often found in groundwater. Although only these two contaminants have been tested and shown to be removed by steel slag, it is believed that the present invention can be used to remove other contaminants typically found in contaminated groundwater. Such contaminants can be categorized into three general groups: Chlorinated organic compounds, metal cations and anions, and inorganic anions. In addition to TCE, such chlorinated organic compounds also include perchloroethylene, 1,1,1-trichloroethane, and their respective breakdown products in groundwater. In addition to hexavalent chromium, other metal cations often found in contaminated groundwater include pentavalent arsenic, trivalent arsenic, hexavalent uranium, and trivalent selenium. Inorganic anions often found in contaminated groundwater include phosphate and nitrate.
As used herein, the phrase xe2x80x9cpurifying contaminated groundwaterxe2x80x9d means that at least some of the contaminants listed above are reduced by at least some extent. Although the mechanism of purification of the contaminants by the steel slag is not clearly known, it is presumed that steel slag serves to reduce metals such as hexavalent chromium and uranium to water insoluble substances and prevent their mobility, thus protecting groundwater. The present invention is particularly directed to reducing hexavalent chromium, and it is believed that the iron present in the steel slag serves to reduce hexavalent chromium. Accordingly, it is preferable in some cases to use a steel slag having a higher concentration of iron oxide, such as higher than 10% or even more preferably higher than 15% in situations where there is a high concentration of hexavalent chromium in the contaminated groundwater. In addition, it is believed that steel slag will adsorb metal ions of arsenic and selenium and therefore prevent their mobility in water solution as well. It is also believed that steel slag can render inorganic anions insoluble in water or adsorb the anions limiting their mobility in water solution. It is also believed that steel slag serves to degrade chlorinated organic substances that are soluble in groundwater. More generally, the presumed mechanisms listed in U.S. Pat. No. 4,377,483, incorporated herein by reference, might also be applicable here as listed below:
A. Adsorptive effect caused by 2CaO.SiO2 and phosphoric acid compounds;
B. Co-precipitation effect caused by Fe;
C. Precipitation effect caused by S;
D. Hydroxide precipitation effect caused by a high pH value (9.5 to 11.5); and
E. Ion substitution effect caused by CaO and MgO.
In implementing the method of the present invention, the well known teachings of permeable reaction wall technology, as used for zero-valent iron filings, are utilized. Some of these teachings are set forth, for example, in an article entitled xe2x80x9cIn situ Treatment of Groundwater: Metal-Enhanced Degradation of Chlorinated Organic Contaminants,xe2x80x9d Robert W. Gillham, M. M. Aral (Ed.), Advances in Groundwater Pollution Control and Remediation, pp. 249-274 (1996) and U.S. Pat. No. 5,266,213 to Gillham, both incorporated herein by reference.
In implementing the method of the present invention, a hydraulic evaluation of the groundwater flow is first made with equal potential lines used to show the flow path of the groundwater. In the hydraulic study, the type of aquifer material is analyzed and the average groundwater velocity is determined along with the typical range of the direction of flow, which might vary seasonally. The groundwater is then sampled at various sites for contaminants to determine the plume of contaminated groundwater. Then, column tests are conducted in the laboratory to determine the half lives of the various contaminants which are desired to be reduced. The number of half lives are determined to reduce the concentration of certain contaminants (e.g., TCE) from the amount present in the contaminated groundwater to the desired amount (such as a drinking level limit). The number of half lives are multiplied by the half life determined from the laboratory tests to provide a residence time, which in turn is multiplied by the flow rate of the groundwater to determine the thickness of the wall.
In analyzing the aquifer, it should be confirmed that the permeability of the wall of steel slag is about the same permeability as the aquifer so that the flow rate does not change dramatically across the permeable barrier. In order to approach the permeability of aquifer, the particle size of the steel slag can be reduced or increased within the ranges given above. As the particle size is increased, the permeability of the steel slag wall increases as well.
In order to place the permeable wall of steel slag in the ground, a ditch of the desired dimensions is excavated and is simply filled with steel slag then typically covered with the aquifer material. The steel slag may be added directly to the ditch with no permeable membrane. Alternatively, a permeable membrane, such as a geotextile material, may be placed within the ditch to separate the steel slag from the aquifer material, although this is not necessary.
After placement, the groundwater is again sampled to ensure that the contaminants have been adequately removed. The sampling should continue periodically over time to confirm that the steel slag remains active. At some point in time, typically a number of years, the steel slag would become spent in that it would no longer purify groundwater to an adequately low level of contaminants. In this event, an excavator can be used to simply remove the spent steel slag and lay some fresh steel slag in its place. Alternatively, a new ditch can be dug adjacent the spent steel slag and filled with fresh steel slag. As described in the ""483 patent, treating groundwater with steel slag has the advantage that the heavy metals once adsorbed do not readily dissolve again so that the spent steel slag can be discarded without being a public hazard. For example, the spent steel slag can readily be solidified with an ordinary Portland cement or a blast furnace cement or a plaster. Alternatively, the spent steel slag can be used as an upper layer stabilizer.
Although steel slag contains certain elements, such as at least one of sulfur, selenium, arsenic, cadmium, lead, copper, and mercury, the form of the steel slag is such that these elements are less likely to leach from steel slag than from iron filings. In addition, steel slag contains calcium and magnesium that are released to aqueous environments which would increase the pH level to 10 or 11 and facilitate the removal of phosphates, arsenates, trivalent chromium, manganese, and aluminum and prevent plugging by bacterial growth. Moreover, steel slag is a glassy (amorphous) coarse material that maintains relatively high permeability (about 4.5xc3x9710xe2x88x922 cm/sec.) regardless of the amount of water passed through it. Finally, steel slag does not absorb carbon dioxide from air or react with carbonates or carbonic acid from aqueous solution to form insoluble carbonates.
The following examples are included to more clearly demonstrate the overall nature of the invention. These examples are exemplary, not restrictive, of the invention.